I just put together a battery system and it works great with the Yaesu FT-8800R. Now I'm trying to calculate usage rates so I can determine how many more batteries I might want to get to build up the system.

The specs for the radio say it consumes 0.5 A on Rx and 8.5 A on Tx. If I assume a 10% Tx duty cycle would this be the right way to calculate power usage?

I understand I can run a deep cycle battery down to about 80% charge, but it is preferable to keep the discharge to 60% or less and recharge as soon as possible afterward. I have a 115 Amp/Hr RV battery, so if I use 60% as a target and 80% as max discharge target ...

Did I do this right? Assuming I am not running anything but the FT-8800R (and maybe charging my cell phone) another 115 A/Hr RV battery bringing total to 230 A/Hrs should be plenty for the 72 hour deployment target in our ARES/RACES plan, yes??

Thanks for checking my math. I am very new to this and want to make sure I understand it.

Your source calculations would be too if your source were perfect. But batteries are sensitive to the current being drawn from them. The more current you draw, the fewer Ah capacity it will have. This can be calculated too, based on numbers provided by the manufacturer, or empirical tests you do. This is known as the Peukert effect, and the merit calculated for your battery is called the Peukert constant.

<http://en.wikipedia.org/wiki/Peukert's_law>

Essentially the Peukert equation takes the specified or measured amp hours for your battery at two different levels, preferably somewhere near the range of currents you anticipate using (which improves the accuracy). Once this is done, a pretty darn accurate approximation of Ah capacity can be calclulated for any other current draw.

All this can be put in a spreadsheet. Put in your loads and duty cycles, your battery particulars and the Peukert formula and you can very accurately predict your run time for any scenario you come up with. One thing to be sure to include is at what voltage your equipment stops functioning. You reach the 80% DOD point right around 10.5V but most equipment won't operate at that low of a voltage. So if it stops functioning at 11.75V for instance (13.8 - 15%, typical equipment spec) you will not be able to use the energy in that battery left below that point (~30%). So that either has to be accomodated by allowing a shorter run time, using a bigger battery, or using a boost converter that lets you run the battery down to the 80% DOD point. BTW, don't limit yourself to 50% or 60%, take it all the way down to 80%. There's no way in typical service you'd ever use up the cycle life of a deep cycle battery, so you might as well get your money's worth.

It's going to work out that the largest variable in your analysis will be accurately characterizing the load. It's easy enough at the outset to just come up with the two static currents and a duty cycle, and for most purposes that'll get you close enough. But it starts to get more fuzzy when you start adding more loads and their duty cycles, then maybe clamp a charger or solar panel on top of that (charge current is also subject to Peukert, as well as a few other variables).

One thing at a time though, play with this for a while and get comfortable with the concept and you can work your way up from there.

One thing I find quite useful is a state of charge meter. You can buy a fancy one with a digital display that counts amp hours but I find that a simple expanded scale voltmeter gives you an instant view of where your battery is at, under any current draw. Because the useable state of charge pretty closely follows the straight line between 10.5V (empty) and 12.75V (full), a voltmeter that covers this range becomes a "gas gauge" for your battery. You can buy one in the form of an analog automotive voltage gauge or make one with a handful of parts. For extra credit you can use a dual needle meter (like an SWR meter has) and know your SOC and dynamic parameters at a glance.

I have looked for a simple, digital voltmeter to two decimal places (I understand the second decimal will fluctuate a bit) and have found them for about $8 and for over $50 with very little in between. This leads me to believe perhaps the cheapies are not suitable, but I don't know if that is true.

Anyone have a digital voltmeter they trust that they can point me to in a link? I'm an electronics know-nothing and unlikely to build anything.

Depends on what you want to use them for. I wouldn't limit myself to just one. They're like screwdrivers and flashlights, you can't have too many.

I have probably a dozen of those $3 harbor freight meters. I think they're great. For what amounts to be the cost of a 9V battery, you get a meter for free. I've had a couple that were off in the weeds but that just gave me a few spare batteries and probes. Buy three and keep the two that agree, HI. Having a bunch of them around means you never have to hunt for one and if you ruin it, you're not out much.

I also have a couple nice logging handheld meters, ones that I won't use testing things that might damage them. By having the cheap meters around, I'm less likely to put the nice ones at risk.

Sears and Radio Shack both have a selection of multimeters $50 and under. I'm sure there's tons of deals online as well.

Oh, I have a multi-meter. I could hook that up, but I thought in your earlier post you were suggesting having one hard-wired to the battery at all times, during charging, use and non-use??? I might get one and mount it in the lid of the battery box where it is always visible.

By the way, the Peukert numbers, whether the load is constant or intermittent, and the fact that I might want to recharge a cell phone or something, nobody answered my question about two of these batteries being sufficient for a 72-hour stint in the field. Powering primarily just the FT-8800 I'm thinking 230 amp/hours of RV deep cycle battery should be plenty, yes? All the niggling details aside, that is. Seems like it to me.

For a leave-it-connected meter I use primarily analog meters, movements that I've found at hamfests or whatever and set up as expanded meters as mentioned above. Most meters you can get the covers off and paste a new face onto, so you can put whatever graphics you want on them. Or quick 'n dirty just mark up the existing face with colored sharpie markers.

You can use a digital meter but under changing loads the numbers dance around, and you have to mentally interpret the numbers into SOC. Plus, they run on an internal battery which means you can't leave it on all the time where with a passive meter they're always working. It's whatever your preference is.

> nobody answered my question about two of these batteries > being sufficient for a 72-hour stint

I don't know the Peukert constant of your batteries but we can assume as RV 12V deep cycle units it will be around 1.25 (calculated per the data sheet of a Trojan 27TMH).

Plugging your load into my Peukert spreadsheet I get a corrected Ah rating for two of these batteries in parallel of 215Ah. But that's down to 10.5V, which you won't get to based on the assumed 11.75V cutoff point.

I would put 11.75V at about the 50% available Ah point, so essentially you're halving the total capacity of the pack, giving you 107.5 useable Ah. At 1.3A average draw into 107.5Ah you get 82 hours. Looks like you've got a little left over to charge a cellphone or two (maybe more, depending on their voltage cutoff) and give you a bit of a margin. You wouldn't get there on a single battery and a boost converter, between Peukert and the additional load and efficiency of the converter it wouldn't make it.

Of course as these are all seat o' pants calculations, the real test is to hook up a load to the subject battery and see what it really does. In practice I've found the battery performance to be quite predictable and repeatable, so once you characterize your specific battery your run time calculations will be as accurate as far as you can quantify the load.

Excellent, thanks for the help with those calcs! I'm just trying to meet my club's and the ARRL's recommendation of planning for a 72 hour deployment in your go-kit planning. Won't be operating 24/7 I'm sure, so if I get one more of these at some point, power is pretty much taken care of.

keep in mind that a connected analog/mechanical volt meter can run a battery dead fairly quickly.. I've found that in 3 to 4 days an automotive type volt meter can take a standard automotive starting battery down to unusable.

I built a station using a 25w transmitter and two 12v 11Ah sealed batteries. I included two power switches so that when it's not in use, I turn off everything, and use an automotive maintenance charger to keep the batteries happy. this is an 'intelligent' automatic charger, _NOT_ a float charger. sealed batteries and float chargers are not a good combination! (float chargers can cause out gassing, and sealed batteries can't out-gas until they blow their safety valves out)

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